1. Field of the Invention
The present invention relates to a polymer electrolyte battery including a positive electrode, a negative electrode and a polymer electrolyte. More particularly, the invention relates to a polymer electrolyte battery with better charge/discharge cycle characteristics attained through improvement in the characteristics of a positive electrode/polymer electrolyte interface and a negative electrode/polymer electrolyte interface.
2. Description of the Related Art
Recently, a non-aqueous electrolyte battery of high electromotive force based on the oxidation/reduction of lithium in the non-aqueous electrolyte solution has been used as one of novel batteries featuring high power and high energy density.
Unfortunately, such a non-aqueous electrolyte battery suffers leakage of the non-aqueous electrolyte solution or degradation of the battery characteristics resulting from reaction between the non-aqueous electrolyte solution and the positive or negative electrode. In this connection, the polymer electrolyte batteries employing the polymer electrolyte have received current attention.
The prior-art polymer electrolyte batteries generally employ a polymer electrolyte composition which comprises a polymer, such as polyethylene oxide and polyvinylidene fluoride, containing a solute of lithium salt, such as LiPF6, or a polymer electrolyte composition which comprises a polymer impregnated with a non-aqueous electrolyte solution containing an organic solvent, such as carbonate, dissolving the above solute therein.
Unfortunately, however, the aforesaid polymer electrolyte batteries suffer such a poor adhesion between the positive or negative electrode and the polymer electrolyte that resistance is increased at the positive electrode/polymer electrolyte interface or the negative electrode/polymer electrolyte interface. This detrimentally degrades the charge/discharge cycle characteristics of the polymer electrolyte batteries.
In a recent attempt to improve the polymer electrolyte battery in the charge/discharge cycle characteristics, a surfactant is added to the polymer electrolyte for reducing the resistance at the interface between the polymer electrolyte and the positive or negative electrode.
However, the addition of the surfactant to the polymer electrolyte involves a fear that the surfactant may form impurities responsible for the deterioration of a positive- or negative-electrode active material, thus conversely lowering the charge/discharge cycle characteristics of the polymer electrolyte battery.
A first object of the invention is to enhance the adhesion between the positive or negative electrode and the polymer electrolyte of the polymer electrolyte battery including the positive electrode, negative electrode and polymer electrolyte, thereby to reduce the resistance at the interface between the positive or negative electrode and the polymer electrolyte.
A second object of the invention is to enhance the ionic conductivity at the interface between the positive or negative electrode and the polymer electrolyte.
A third object of the invention is to improve the charge/discharge cycle characteristics of the polymer electrolyte battery.
In the polymer electrolyte battery including the positive electrode, negative electrode and polymer electrolyte in accordance with the invention, at least one of the positive electrode and the negative electrode is formed with an inorganic amorphous solid electrolyte film at its interface with the polymer electrolyte. The amorphous structure is defined herein to be a non-crystalline state resulting from the loss of crystalline lattice and may be produced by deposition or sputtering.
As suggested by the polymer electrolyte battery of the invention, if the positive or negative electrode is formed with the inorganic amorphous solid electrolyte film at its interface with the polymer electrolyte, the adhesion between the polymer electrolyte and the positive or negative electrode is increased by this inorganic solid electrolyte film. In addition, the inorganic solid electrolyte film itself has the ionic conductivity. Hence, the resistance at the interface between the polymer electrolyte and the positive or negative electrode is decreased. As a result, the interface between the polymer electrolyte and the positive or negative electrode is improved in the ionic conductivity so that uniform cell reactions take place in repeated charge/discharge processes, leading to the improved charge/discharge cycle characteristics of the polymer electrolyte battery.
As an inorganic solid electrolyte for forming the aforesaid inorganic amorphous solid electrolyte film, there may be used at least one material selected from the group consisting of, for example, Li3N, LiTi2(PO4)3, Li-xcex2Al2O3, LiI, LiIxe2x80x94Li2Sxe2x80x94P2O5, LiIxe2x80x94Li2Sxe2x80x94B2S3, LiIxe2x80x94Li3Nxe2x80x94LiOH, Li2Oxe2x80x94B2O3, Li2Oxe2x80x94V2O3xe2x80x94SiO2, LiTaO3 and the like.
If the inorganic solid electrolyte film on the positive or negative electrode has an excessive thickness, the inorganic solid electrolyte film has such a great resistance as to lower the ionic conductivity at the interface between the positive or negative electrode and the polymer electrolyte. For this reason, the inorganic solid electrolyte film preferably has a thickness of 10 xcexcm or less.
The inventive polymer electrolyte battery is characterized in that at least one of the positive electrode and the negative electrode is formed with the aforesaid inorganic amorphous solid electrolyte film at its interface with the polymer electrolyte and does not particularly limit the materials for forming the positive electrode, negative electrode and polymer electrolyte. The inventive polymer electrolyte battery may employ any known materials generally used in the art.
As the aforesaid polymer electrolyte, there may be used a solid-state polymer electrolyte comprised of a polymer containing a solute, or a gelated polymer electrolyte comprised of a polymer impregnated with a non-aqueous electrolyte solution containing a solvent dissolving a solute therein.
Any of the know materials generally used in the art may be employed as the aforesaid polymer. Examples of a suitable polymer include those having a molecular weight of 5000 to 500000 such as polyethylene glycol methacrylate, polyethylene oxide, polyethylene-imine, polyvinylidene fluoride, polyacrylonitrile and the like. In particular, the use of such a polymer as a polyethylene oxide and a polyethylene-imine results in an even greater improvement of the charge/discharge cycle characteristics of the polymer electrolyte battery. A conceivable reason for this is because the polymer comprised of a polyethylene oxide or a polyethylene-imine contributes to an even greater adhesion between the polymer electrolyte and the inorganic solid electrolyte film, thereby further reducting the resistance at the interface between the positive or negative electrode and the polymer electrolyte.
Any of the known solutes generally used in the art may be used as the above solute. Examples of a suitable solute include lithium compounds such as LiPF6, LiBF4, LiN(C2F5SO2)2, LiAsF6, LiSbF6, LiAlF4, LiGaF4, LiInF4, LiClO4, LiN(CF3SO2)2, LiCF3SO3, LiSiF6, LiN(CF3SO2) (C4F9SO2) and the like.
Any of the known solvents generally used in the art may be used for dissolving the aforesaid solute in the preparation of the gelated polymer electrolyte comprising the aforesaid polymer impregnated with the non-aqueous electrolyte solution. Examples of a suitable solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane and the like.
Any of the known positive-electrode materials generally employed in the art may be used for forming the positive electrode of the inventive polymer electrolyte battery. Examples of a suitable material include lithium-containing transition metal oxides such as lithium-containing manganese oxides, lithium-containing cobalt oxides, lithium-containing vanadium oxides, lithium-containing nickel oxides, lithium-containing iron oxides, lithium-containing chromium oxides, lithium-containing titanium oxides and the like.
Any of the known negative-electrode materials generally employed in the art may be used for forming the aforesaid negative electrode. Examples of a suitable material include a lithium metal; lithium alloys such as Lixe2x80x94Al, Lixe2x80x94In, Lixe2x80x94Sn, Lixe2x80x94Pb, Lixe2x80x94Bi, Lixe2x80x94Ga, Lixe2x80x94Sr, Lixe2x80x94Si, Lixe2x80x94Zn, Lixe2x80x94Cd, Lixe2x80x94Ca and Lixe2x80x94Ba; carbon materials capable of absorbing/desorbing lithium ions such as graphites, cokes and sintered organic substances; and metal oxides having lower potentials than the positive-electrode material, such as Li4Ti5O12, TiO2, Nb2O5, Fe2O3, MoO2, MoO3, WO2, WO3, SnO2, SnO, SiO2 and SiO. Particularly, the use of the carbon material as the negative-electrode material results in an even greater improvement of the charge/discharge cycle characteristics of the polymer electrolyte battery. A conceivable reason for this is because if the aforesaid inorganic amorphous solid electrolyte film is formed on the negative electrode including the carbon material with a great surface area, this inorganic solid electrolyte film provides a greater surface on which the negative electrode adheres to the polymer electrolyte, thereby further increasing the ionic conductivity at the interface between the negative electrode and the polymer electrolyte.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention.